The present invention relates to improvements in the field of reuseable or squeeze-out carbons and ribbons.
Reuseable or multiple use carbons, invented over twenty years ago as improvements over conventional hot-melt wax carbons, are produced by mixing a resinous binder material dissolved in a major amount of one or more volatile solvents or vehicles, and an oily ink which is incompatible with said resinous binder material, coating said mixture onto a flexible foundation and evaporating the solvents or vehicles to form a cohesive, nonadhesive porous network of said resin containing said ink within the pores thereof.
Such reuseable carbons represented an improvement over conventional hot-melt wax carbons in several regards. Firstly, they could be reused many times, with only a gradual weakening of the color of the images with each repeated use. Conventional hot-melt wax carbons have high adhesion and low cohesion properties and can be reused only a few times. The soft wax compositions are frangible and transfer as a solid mass in image form under each imaging pressure, including the wax binder, due to the low cohesive properties thereof. The intensity of the formed images is sharply reduced after the first use and is noticeably nonuniform in the case of images formed from areas of the carbon comprising overlapping new and used areas. Modern high-speed printers or typewriters operate on the so-called "multistrike" principal in order to reduce the length of ribbon used therein. Thus, the ribbon advancement speed is slowed so that each impact of the type face overlaps with one or more prior impacts. Thus, each typed or printed image is transferred from an area of the ribbon which has already transferred ink to one or more previous images. A conventional hot-melt typewriter carbon or film ribbin is unsatisfactory for such use because too large a portion of the ink composition is transferred under a single impact pressure. Subsequent images formed from overlapping areas of the same carbon or ribbon are clearly visible spotty or discontinuous. Multistrike use requires that at least about three, and preferably five or more, original-appearing images be produced fom each overstruck area of the carbon or ribbon. Conventional reuseable squeeze-out solvent carbons fill this requirement whereas conventional hot-melt typewriter carbons do not.
Secondly, reuseable or squeeze-out carbons produce images which are smear-resistant and clean-to-the-touch since such images comprise fluid ink which is absorbed by the copy paper. Conventional hot-melt wax carbons produce images which can be smeared and are dirty-to-the-touch since they comprise large amounts of softened or plasticized pigmented wax binder material in the form of solid raised images sitting on the surface of the copy paper.
While reuseable, squeeze-out carbons and ribbons represent a substantial advance in the art, they also have disadvantages. The volatile coating solvents or vehicles are expensive and are lost during the manufacturing process unless expensive solvent-recovery equipment is employed. Many of the conventional coating solvents or vehicles can no longer be used because they are classified as pollutants and/or as carcinogens or toxic chemicals. The solvent-coating equipment is expensive and requires a large amount of floor space due to the length of the necessary drying tunnel, etc. Also, solvent-coating processes cannot be carried out in certain areas, building or neighborhoods where precluded by fire laws designed for protection against fire and explosion.
It has been proposed to produce reuseable, squeeze-out carbons from plastisol formulations which comprise liquid resin binder-plasticizer materials which fuse at elevated temperatures to form microporous, ink-releasing coatings. While such formulations are useful for making stamp pads and ink rolls, they are unsuitable for use on thin paper or plastic films due to the high temperature of fusion.
In addition to the foregoing problems and disadvantages, some conventional reuseable, squeeze-out carbons and ribbons have a relatively high threshold of imaging pressure which must be exceeded before ink is exuded to a copy sheet. Some conventional typing and printing machines, such as the so-called Daisy wheel machine, exert a relatively low impact pressure. Other typing and printing machines are adjustable relative to their impact pressure. It is desirable for the carbons and ribbons to have a low threshold of imaging pressure, for use on machines having or adjusted to have relatively low impact pressures since such machines operate more quietly and generate less heat and wear than machines which exert higher impact pressures.
Also, conventional squeeze-out solvent carbons generally contain solid pigments rather than dissolved dyes, because of the relatively poor dye-dissolving ability of the resin-incompatible oleaginous ink vehicles therein, which makes it difficult to produce such carbons and ribbons having a variety of different-colored inks.
Conventional squeeze-out solvent carbons also have relatively poor transferability to copy sheets having rough or porous surfaces since the exuded ink only wets or stains the portions of the copy sheet contacted thereby. Thus, images formed on rough surfaces may be spotty, discontinuous and less clear or sharp than desired.
Finally, conventional solvent-coated, resin-base, reuseable carbons and ribbons require the presence of an adhesive or soluble bonding layer between the microporous ink layer and the plastic film foundation in order to prevent the ink layer from separating from the foundation during repeated reuse. Such bonding layers increase the cost and thickness of the carbons and ribbons and necessitate an additional coating step in the manufacturing process.